In recent years, electronics technology advances have allowed higher performance, miniaturization, and reduction in weight and power consumption in electronic equipment. Consequently, various household portable appliances have been developed and put into practical use and their marketplace has been rapidly expanded. Representative examples thereof include a cam-corder, a notebook-type personal computer, and a portable telephone. This trend has created an increasing demand for further miniaturization and weight reduction as well as longer time duration in such appliances. To comply with such requirements, a lithium rechargeable battery offering a longer working life and a higher energy density, typified by a lithium-ion rechargeable battery, has been developed vigorously and put into wider use as a built-in driving power source for the appliances.
A lithium-ion rechargeable battery has the advantage over other commercially available batteries in that it offers not only sufficiently high energy density per unit volume (used as an index of miniaturization of a battery) but also significantly high energy density per unit weight (used as an index of weight reduction of a battery). The energy density of a battery depends mainly on battery active material of positive and negative electrodes constituting its element for electromotive force. However, miniaturization and weight reduction of a battery case for housing the element for electromotive force also plays an important role in determining the energy density. That is, a battery case which has a thinner wall than same-outside-shape battery cases is capable of accommodating a larger amount of battery active material. This helps increase the volume energy density of then battery. Moreover, by forming a battery case from a light-weight material, it is possible to reduce the weight of the entire battery and thus to increase its weight energy density.
As a natural consequence of the above-described trend in battery development, a prismatic battery which employs a thin-walled prismatic battery case is highly rated in terms of suitability for use in slim appliances and high space efficiency. Conventionally, as a method for manufacturing a prismatic battery case, so-called transfer drawing has been customarily adopted. In this processing method, by repeating deep drawing and stamping over 10 to 13 times using a transfer press machine, a battery case having a substantially rectangular cross section is fabricated.
However, in a prismatic battery case manufacturing method based on the transfer drawing, deep drawing and stamping need to be repeated over a dozen or so times. This leads to poor productivity (for example, 20 pieces of articles per minute). Moreover, according to the transfer drawing, reduction in a wall thickness of a battery case material, which is necessary to obtain higher volume energy density and thus higher capacity, is achieved by repeating deep drawing. Thus, a prismatic battery case realized by the transfer drawing suffers from lower strength and thus fails to provide a desired pressure withstanding strength when operated as a battery system. In particular, a prismatic battery case unlike a cylindrical battery case which is stable in shape under increased internal pressure of a battery, becomes distended like a barrel to approach a configuratively stable cylindrical shape. This might cause leakage of electrolyte and damage to the appliance.
Meanwhile, as a method for manufacturing a battery case for use in a cylindrical battery, DI method is used (see Japanese Examined Patent Publication No. Hei. 7-99686). With this method, a battery case is fabricated that, despite being made thin to enhance the volume energy density, provides desired pressure withstanding strength with higher productivity. In this DI method, a cup-like intermediate component fabricated by deep drawing using a press machine is subjected to drawing and stamping successively at a time. As compared with the transfer drawing method, the DI method has the following advantages: higher productivity is attained by reducing the number of process steps; the weight of a battery case is reduced by reducing the wall thickness of its side wall; battery energy density improves with an increase in battery capacity; and stress corrosion is suppressed. Thus, the DI method is coming into wider and wider use in manufacturing a battery case used for a cylindrical battery.
Hence, it can be considered that the above-described DI method is applied to the manufacture of a prismatic battery case. In this case, however, the following problem arises. In a case where a cylindrical battery case is fabricated by the DI method, during the DI processing, a cup-like intermediate component having a circular cross section is simply processed into a similar-shape battery case, i.e. into a battery case having a circular cross section. Accordingly, in an ironing step of the DI processing, the thickness of the entire side wall is evenly reduced. This allows the material to flow evenly during the processing, thereby achieving smooth deformation. By contrast, where a prismatic battery case is fabricated by the DI method, during the DI processing, a cup-like intermediate component having a circular cross section needs to be processed into a different-shape battery case, i.e. into a battery case having a substantially rectangular cross section. Accordingly, the material flows unevenly during the processing and is thus processed unstably, with the result that cracking, rupture, or distortion tends to occur particularly in the shorter-side plate of the battery case which is smaller in area.
For this reason, it has conventionally been impossible to fabricate a prismatic battery case by the DI method. Resultantly, a prismatic battery case is generally fabricated by the above-described transfer drawing method or impact molding using an aluminum material. In either case, however, the productivity is extremely unfavorable, and what is worse, it is impossible to form a battery case having a strength high enough to prevent deformation due to increased battery internal pressure without sacrificing its slimness and lightness. This makes it impossible to attain satisfactory volume energy density and weight energy density.
As another method for manufacturing a prismatic battery case, Japanese Laid-open Patent Publication No. Hei 6-333541 proposes a technique whereby a rectangular barrel and a bottom plate are separately formed and then the bottom plate is air-tightly bonded to the bottom portion of the rectangular barrel by laser welding. In this method, however, the number of process steps is almost the same as that in the transfer drawing method. Moreover, the method requires time-consuming operations including accurate positioning of the rectangular barrel and the bottom plate and laser welding process. This makes it impossible to attain improved productivity. Further, a prismatic battery case realized by the method fails to satisfy two mutually contradictory requirements, namely, offering higher energy density attained as a result of reduction in thickness and weight and offering pressure withstanding strength high enough to prevent deformation due to increased battery internal pressure.
The present invention has been made in the light of the above-described problems with conventional art, and accordingly its object is to provide a prismatic battery case which offers higher energy density as well as desired pressure withstanding strength, and a method for manufacturing the prismatic battery case by DI method.